INSIGHTS FOR COMPLETE DEGRADATION OF NON-STARCH POLYSACCHARIDES IN POULTRY DIETS

Non-starch polysaccharides (NSP’s) are the major components of dietary fiber primarily composed of cellulose and non-cellulosic polysaccharides. In cereal grains, including corn, the non-cellulosic polysaccharides consist of arabinoxylans and β-glucans while in soybean and canola meals, arabinans, arabinogalactans, galactans, galactomannans, mannans and pectic polysaccharides predominate. Although there are many ways of classifying NSP’s in poultry nutrition, mostly the classification is based on the solubility in water namely water soluble and water insoluble NSP’s. Ingestion of soluble NSP’s like arabinoxylans, β- glucans and pectins etc., increases the digesta viscosity in intestine of broilers by directly interacting with water molecules. At higher concentrations, the molecules of the NSP’s interact themselves and become entangled in a network further increasing the viscosity. Because of the formation of networks with water, water holding capacity of soluble NSP’s are relatively high compared to insoluble NSP’s. The insoluble NSP’s like cellulose pass through the gastrointestinal tract unchanged and are biologically inert. The presence of NSP’s in the feed raw materials reduces the digestibility and limits the apparent digestible energy (AME).

Biological and physiological effects

The increased viscosity due to soluble NSP component reduces physical contact between endogenous enzymes and nutrients by acting as a barrier which decreases movement of enzymes and substrate molecules. This affects digestibility of starch, proteins, lipids and leads to reduced performance of broilers because of impaired nutrient utilization. Also increased viscosity reduces rate of passage of digesta and increases retention time leading to stasis for long time stimulating secretion of digestive juices more than the required amount. This leads to increased endogenous nitrogen loss besides increasing thickness of unstirred water layer adjacent to intestinal mucosa, thus reducing diffusion of nutrients for absorption. As soluble NSP’s increase the viscosity of the digesta, they also lead to changes in the physiology and the ecosystem of gut. This effect is mainly related to a slower digesta rate. A slow moving digesta with low oxygen tension in the small intestine could provide a relatively stable environment where fermentative microflora can establish. This increased bacterial population in turn compete for nutrients with host, causing irritation and thickening of gut mucosa, additionally increasing proliferation of enterocytes which changes the morphology of villi. The implication of increased dietary soluble NSP in the onset of other diseases, such as necrotic enteritis in poultry has also been documented.

It has been clearly demonstrated that the presence of NSP’s in broiler diets affects the digestibility of starch, protein and lipids. Lipid digestibility being predominantly depressed than starch and protein. The probable reason for this effect of NSP’s on lipid utilization may be related to increased digesta viscosity resulting from the soluble NSP’s which reduces the constant interaction between potential nutrients and the digestive secretions like lipid digesting enzymes, bile salts etc. Additionally, the viscous NSP’s are capable of entrapping the bile salts thereby limiting fat digestion and absorption. The increase in intestinal viscosity, altered gut microbial profile, reduced nutrient digestibility and absorption caused by the soluble NSP’s leads to decrease in growth rate and feed efficiency.

Scope of NSPases

Presence of NSP’s in the diet may be of harmful consequence to the health and productivity of the birds as they negatively impact or reduce nutrient utilization and absorption. However, monogastric animals including poultry lack the ability to hydrolyze the NSP’s present in the feed ingredients. Major parts of NSP’s (soluble and insoluble) remain intact because of the lack of suitable enzyme activities within the digestive tracts of the animals posing a serious threat to the productive performance of the modern growing poultry strains. To prevent the negative impact of NSP’s, exogenous enzyme supplementation has been found to be a very effective tool for poultry producers to improve performance and nutrient utilization. Various workers have documented that addition of NSP enzymes have led to a beneficial effect on the performance in poultry. Positive impact of NSP enzymes on animal performance can be summarized due to their effect on the intestinal viscosity, modification of gut microflora and improvement in nutrient digestibility primarily by their impact on the fiber portion of the ingredients. Dietary addition of exogenous enzymes particularly NSPases has gained much importance in recent past as usage of an unconventional feed ingredients in feed formulations have increased. These ingredients like rap seed, Dried Distilled Grain with Solubles (DDGS), wheat etc., have lower digestibility than corn or soya and contain some of anti-nutritional factors. Exogenous enzymes are being extensively researched in both single enzyme inclusion and in a so called “cocktail” form involving suitable enzyme combinations. Judicious use of dietary enzyme addition can be an effective method to decrease disparity between flocks and prevent nutritive inconsistencies between feed sources which can otherwise lead to economic losses. The addition of exogenous NSP enzymes break open the intact cell walls, denature the chemical bonds and thereby increase the availability of nutrients to the animal. Exogenous enzymes function to degrade NSP present in the diet by breaking the fiber chains in the cell walls into smaller fragments. By breaking down the cell wall of grains, carbohydrase inclusion has shown to decrease intestinal viscosity, increase digestibility and performance.

As presence of ingredients in feed formulations is not consistent and varies across the world depending upon the locally available ingredients, many types of the grains and oil seed cakes are used in poultry diets. These feed ingredients contain a variety of NSP’s like arbinoxylans, mannans, galactans etc. Hence use of a product containing a combination or “cocktail” of enzymes ranging in their specificity and action can be the most effective practice in degradation of NSP’s, although improvements in degradation of NSP’s have been observed with individual enzyme inclusion such as xylanases. Enzymes included in NSPase cocktails may vary, however enzymes such as xylanase, β-glucanase, mannanases, cellulase and pectinase are often included to increase their spectrum for substrates leading to better degradation of most of available non starch polysaccharides. The inclusion of carbohydrases in diets based on wheat, rye and barley show more consistent improvements in performance due to higher NSP content when compared to corn due to lesser availability of NSP substrates in corn. Exogenous enzymes supplemented in feed must be able to tolerate the changes in pH of the poultry gastrointestinal tract before they can reach the site of degradation, as enzyme activity is also a function of the pH of the medium in addition to temperature and substrate availability. Additionally, they may be exposed to the high temperatures and difficult processes conditions which are associated with the production of the poultry feeds. Furthermore, the substrate interaction time for these enzymes which is a crucial feature for the efficacy of enzyme is limited, due to the short rate of passage time in poultry. So to be effective the enzyme must act and degrade its target substrate within a reasonable time frame.

Effects of NSP enzymes on performance

Positive impact on the growth and performance by inclusion of exogenous enzymes was reported to be inconsistent in corn-soy based diets, which forms the primary cereal grains used in diet formulation in poultry industry. In recent years however, publications from various workers have found that there was more consistent improvement with dietary exogenous enzyme inclusion in corn soya diets which have a lower NSP content when compared to wheat and rye based diets. The starchy endosperm of corn is constructed mainly of small amounts of cellulose encrusted with hemicellulose, the bulk of which is arabinoxylan with lesser amounts of β-glucans and mannans. Utilizing enzymes that aim at these substrates will allow the animal to efficiently degrade the cell walls and release encapsulated starch and protein. Mannanases and pectinases specifically target the soya portion of the diet. Appreciable improvements in growth and feed conversion have been associated with exogenous addition of NSP enzyme combinations on corn soya diets due to improvements in the nutrient utilization, dry matter retention, apparent ileal digestibility coefficients of dry matter, nitrogen, and energy. Broadly, addition of exogenous NSP enzymes either alone or in combination resulted in an increase in AME values ranging from 0.5% to 11%. Additionally, the inclusion of NSPases not only resulted in an improvement in AME of the diet alone but also contributes to a definite positive impact to digestibility of fat, crude protein and starch, particularly there is improved utilization of energy from fats since fibrous constituents in feed bind with bile acids which prevents fat emulsification and increases size of fat droplets. Furthermore, due to reduction in viscosity caused by presence of soluble NSP’s, there is an easy access of proteases and lipases with their respective substrates in the ingesta leading to better digestibility and utilization. Also plant cell walls contain protein in their matrix, so degradation of NSP’s makes the amino acids entrapped in fiber available for digestion and absorption. The beneficial effect of the NSPases are not only limited to improved nutrient utilization but also their positive impact on their gut health and ecology. Since enzymes improve the nutrient utilization in the proximal gut resulting in reduction of fermentable substrates reaching to hind gut which decreases the harmful microbial population and also reduces the viscosity in the gut, allows the ingesta to pass in optimum time frame lesser than the time required by the bacteria to thrive in the gut. As anti-nutritional effect of NSP’s is also directly or indirectly mediated by their impact on gut microlflora, addition of enzymes has been found to reduce the pathological bacterial population in the gut by restricting substrates needed for their growth. Breakdown of plant cell wall carbohydrates by NSP enzymes can eventually lead to production of short-chain oligosaccharides (di- and trisaccharides). These oligosaccharides are known to be a substrate for bacterial fermentation, potentially positively altering bacterial populations within the gut.

Holistic approach of breakdown of NSP’s

Commercial NSP preparations which degrade the main chain of the NSP’s especially xylanase, cellulase and glucanase play an important role in improving their digestibility. This approach though effective, does not completely break down the more complex structure involving the cross linkages containing ferulic acid which cross links the cell walls of polymers. Hydroxycinnamic acids (HCA), notably ferulic acid (FA) and its diferulic form (DiFA) are known to play a significant role in interconnecting cell-wall polymers. Interactions between xylans and polymers like β-glucans or cellulose also contribute to the cohesiveness of the wall network. The closely interconnected cell-wall network would then possibly impair arabinoxylan susceptibility to endoxylanase by limiting enzyme contact and mobility. Ferulic acid is the major acid found esterified to carbohydrates in the plant cell wall. Ferulic acids are linked to polysaccharides, including glucuronoarabinoxylans (GAX’s), xyloglucans and pectins, through ester linkages.

Comprehensive hydrolysis of the hemicellulose fraction of the NSP group necessitates two groups of enzymes: one which cleaves the xylan main chain (endoxylanases) and the accessory enzymes that remove the side chains and break the crosslinks between xylan and other plant polymers. The latter group consists of α-L-arabinofuranosidase, α-glucuronidase, acetyl xylan esterases, and feruloyl esterase. Among the accessory enzymes, feruloyl esterases play a key role in enhancing the accessibility of enzymes and subsequent hydrolysis of hemicellulose fibers by removing the ferulic acid side chains and crosslinks. The enzyme cleaves ester bonds between hydroxycinnamic acids esterified to arabinoxylans (AX’s) and certain pectins present in plant cell walls.

So more innovative and inclusive approach involves an enzyme preparation containing a blend of non-starch polysaccharide degrading enzymes including xylanase, mananase, beta gluconase and ferulic acid esterase (FAE) which aids to break the ferulic cross linkages in the plant cell wall. FAE has the ability to hydrolyze the ester bond between the xylan polysaccharide and the ferulate or diferulates present in the plant cell walls. FAE was observed to be synergistic with xylanase, causing more degradation of arbinoxylans and improved fiber digestion as depicted in the graphs below.